Best representing the prior art in the field of the present invention are commonly owned U.S. Pat. No. 4,140,462 to Charles C. Thompson and U.S. Pat. No. 4,207,040 to Robert A. Metcalfe et al. In both of these patents the rotary atomization apparatus comprises a disk means fixedly mounted to the top of a drive shaft. The disk means is formed of a lower body portion and an upper disk. The upper disk is metal and has a concave, dish-shaped recess in its upper surface onto which a layer of ceramic material is disposed. A stream of molten metal is poured onto the center of the rotating ceramic surface of the upper disk and is broken up into fine droplets which are flung radially outwardly into a quenching chamber which rapidly cools them in a manner well known in the art and more completely described in commonly owned U.S. Pat. Nos. 4,025,249 and 4,053,264, incorporated herein by reference.
Returning to the Thompson patent and the Metcalfe et al. patent, it can be seen that the upper disk, lower body portion, drive shaft, and a generally cylindrical adapter member define a plurality of axially concentric, cylindrical, nesting elements. The lower body portion is fixed relative to the shaft by a plurality of radial bolts which pass through the body portion and through the intervening cylindrical adapter member directly into the shaft. The upper disk is secured to the lower body portion by means of an internally threaded hold down nut which threads onto the external surface of the lower body portion. The top of the hold down nut has an inwardly extending annular flange which engages the top surface of a radially extending flange of the upper disk. As the hold down nut is threaded onto the lower body portion it pulls the upper disk toward and against the lower body portion for the purpose of preventing relative motion therebetween.
There are several disadvantages with the foregoing arrangement. For example, the hold down nut can loosen due to centrifugal forces, vibration and differential thermal expansions. The upper disk can rotate relative to the lower body portion during assembly which can damage the O-ring seals provided therebetween for preventing cooling fluid from leaking from the coolant cavity within the rotating disk means. It is difficult to torque the hold down nut since there is no reaction torque, and there is no apparent way to lock the nut into position. Also, the hold down nut, upper disk, and lower body portions are not keyed to each other such that rebalancing would be required if the apparatus were disassembled. Finally, the apparatus of the Thompson and Metcalfe et al patents has been operated at RPMs no higher than about 35,000 RPM. It is desirable, however, to be able to spin the rotary atomization apparatus at about 50,000 RPM or even higher; and this is not within the capability of the prior art.